Oracle Proprietary Join ANSI/ISO SQL: 1999 Equivalent Cartesian Product Cross Join

Database Programming with SQL 7-1 Oracle Equijoin and Cartesian Product

In this lesson, you will learn to: • Name the Oracle proprietary joins and their ANSI/ISO SQL: 99 counterparts • Construct and execute a SELECT statement that results in a Cartesian product • Construct and execute SELECT statements to access data from more than one table using an equijoin • Construct and execute SELECT statements that add search conditions using the AND operator • Apply the rule for using table aliases in a join statement

• The previous section looked at querying and returning data from more than one table in a relational database using ANSI/ISO SQL: 99 syntax. • Legacy versions of Oracle databases required joins to use Oracle Proprietary join syntax, and many of these older databases are still in use. • This lesson introduces Oracle Proprietary join syntax for Equijoins and Cartesian Product, and their ANSI/ISO SQL: 99 counterparts.

• The two sets of commands or syntax which can be used to make connections between tables in a database: – Oracle proprietary joins – ANSI/ISO SQL: 99 compliant standard joins

• Comparing Oracle Proprietary Joins with ANSI/ISO SQL: 1999 Joins Oracle Proprietary Join ANSI/ISO SQL: 1999 Equivalent Cartesian Product Cross Join

NATURAL JOIN

JOIN USING clause Equijoin

JOIN ON clause (if the equality operator is used)

Non-equijoin ON clause

• To query data from more than one table using the Oracle proprietary syntax, use a join condition in the WHERE clause. • The basic format of a join statement is:

SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2;

• Imagine the problem arising from having two students in the same class with the same last name. • When needing to speak to "Jackson," the teacher clarifies which "Jackson" by prefacing the last name with the first name. • To make it easier to read a Join statement and to speed up database access, it is good practice to preface the column name with the table name.

SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2;

• This is called "qualifying your columns." • The combination of table name and column name helps eliminate ambiguous names when two tables contain a column with the same column name. • When the same column name appears in both tables, the column name must be prefaced with the name of the table.

• To qualify the columns, you use the syntax tablename.columnname as shown in the example below.

SELECT table1.column, table2.column FROM table1, table2 WHERE table1.column1 = table2.column2;

• Sometimes called a "simple" or "inner" join, an equijoin is a table join that combines rows that have the same values for the specified columns. • An equijion is equavalent to ANSI: – NATURAL JOIN – JOIN USING – JOIN ON (when the join condition uses "=") • The next slide demonstrates the what, where and how required to join the tables.

• What? The SELECT clause specifies the column names to display. • Where? The FROM clause specifies the tables that the database must access, separated by commas. • How? The WHERE clause specifies how the tables are to be joined. • An Equijoin uses the equals operator to specify the join condition.

SELECT employees.last_name, employees.job_id, jobs.job_title What? FROM employees, jobs Where? WHERE employees.job_id = jobs.job_id; How?

LAST_NAME JOB_ID JOB_TITLE Gietz AC_ACCOUNT Public Accountant Higgins AC_MGR Accounting Manager Whalen AD_ASST Administration Assistant King AD_PRES President Kochhar AD_VP Administration Vice President De Haan AD_VP Administration Vice President Ernst IT_PROG Programmer Hunold IT_PROG Programmer … … …

• Another example: SELECT employees.last_name, departments.department_name FROM employees, departments WHERE employees.department_id = departments.department_id;

LAST_NAME DEPARTMENT_NAME Whalen Administration Hartstein Marketing Fay Marketing Davies Shipping Vargas Shipping Rajs Shipping Mourgos Shipping … …

• If two tables in a join query have no join condition specified in the WHERE clause or the join condition is invalid, the Oracle Server returns the Cartesian product of the two tables. • This is a combination of each row of one table with each row of the other. • A Cartesian product is equivalent to an ANSI CROSS JOIN. • To avoid a Cartesian product, always include a valid join condition in a WHERE clause.

• In this query, the join condition has been omitted:

SELECT employees.last_name, departments.department_name FROM employees, departments;

LAST_NAME DEPARTMENT_NAME

Abel Administration

Davies Administration

De Haan Administration

Ernst Administration

Fay Administration

Gietz Administration

Grant Administration

… …

• As with single-table queries, the WHERE clause can be used to restrict the rows considered in one or more tables of the join. • The query shown uses the AND operator to restrict the rows returned.

SELECT employees.last_name, employees.job_id, jobs.job_title FROM employees, jobs WHERE employees.job_id = jobs.job_id AND employees.department_id = 80;

LAST_NAME JOB_ID JOB_TITLE Zlotkey SA_MAN Sales Manager Taylor SA_REP Sales Representative Abel SA_REP Sales Representative

• Working with lengthy column and table names can be cumbersome. • Fortunately, there is a way to shorten the syntax using aliases. • To distinguish columns that have identical names but reside in different tables, use table aliases. • A table alias is similar to a column alias; it renames an object within a statement. • It is created by entering the new name for the table just after the table name in the from-clause.

• Table aliases are used in the query below. SELECT last_name, e.job_id, job_title FROM employees e, jobs j WHERE e.job_id = j.job_id AND department_id = 80;

LAST_NAME JOB_ID JOB_TITLE Zlotkey SA_MAN Sales Manager Sales Taylor SA_REP Representative Sales Abel SA_REP Representative • When column names are not duplicated between two tables, you do not need to add the table name or alias to the column name.

• If a table alias is used in the FROM clause, then that table alias must be substituted for the table name throughout the SELECT statement. • Using the name of a table in the SELECT clause that has been given an alias in the FROM clause will result in an error.

SELECT last_name, employees.job_id, job_title FROM employees e, jobs j WHERE e.job_id = j.job_id AND department_id = 80;

• If you wanted to join three tables together, how many joins would it take? • How many bridges are needed to join three islands? • To join three tables, you need to add another join condition to the WHERE clause using the AND operator

Table 1 Table 2 Table 3

• Suppose we need a report of our employees and the city where their department is located? • We need to join three tables: employees, departments and locations.

SELECT last_name, city FROM employees e, departments d, locations l WHERE e.department_id = d.department_id LAST_NAME CITY AND d.location_id = l.location_id Abel Oxford South San Davies Francisco De Haan Seattle Ernst Southlake … …

Key terms used in this lesson included: • Alias • Cartesian Product • Equijoin • Join Conditions • Proprietary Join

In this lesson you have learned to: • Name the Oracle proprietary joins and their ANSI/ISO SQL: 99 counterparts • Construct and execute a SELECT statement that results in a Cartesian product • Construct and execute SELECT statements to access data from more than one table using an equijoin • Construct and execute SELECT statements that add search conditions using the AND operator • Apply the rule for using table aliases in a join statement